Plasma processing has been widely used in the semiconductor and other industries for many decades. Plasma processing is used for tasks such as cleaning, etching, milling, and deposition. More recently, plasma processing has been used for doping. Plasma assisted doping (PLAD) or sometimes referred to as plasma immersion ion implantation (PIII) have been used to meet the doping requirements of some modern electronic and optical devices. Plasma doping is different from the conventional beam-line ion implantation systems that accelerate ions with an electric field and then filter the ions according to their mass-to-charge ratio to select the desired ions for implantation. Unlike the conventional beam-line ion implantation systems, PLAD systems immerse the substrate in a plasma containing dopant ions and bias the substrate with a series of negative voltage pulses. The electric field within the plasma sheath accelerates ions toward the substrate thereby implanting the ions into the surface of the substrate.
Plasma doping systems for the semiconductor industry generally require a very high degree of process control. Conventional beam-line ion implantation systems that are widely used in the semiconductor industry have excellent process control and also excellent run-to-run uniformity. Conventional beam-line ion implantation systems provide highly uniform doping across the entire surface of state-of-the-art semiconductor substrates.
In general, the process control of PLAD systems is not as good as conventional beam-line ion implantation systems. In many plasma doping systems, charge tends to accumulate on the substrate being plasma doped. This charge build-up can result in the development of a relatively high potential voltage on the substrate that can cause unacceptable doping non-uniformities and arcing, which can result in device damage. Additionally, the composition of the plasma can affect the resulting process step. For example, a large number of inert gas ions may cause more damage to the substrate than desired. In addition, the electron temperature may alter the desired numbers of ions in the plasma.